Yeast cells having reductive TCA pathway from pyruvate to succinate and overexpressing an exogenous NAD(P)+ transhydrogenase enzyme

ABSTRACT

Yeast cells having a reductive TCA pathway from pyruvate or phosphoenolpyruvate to succinate, and which include at least one exogenous gene overexpressing an enzyme in that pathway, further contain an exogenous transhydrogenase gene.

This application is a divisional of U.S. patent application Ser. No.14/416,633, filed Jan. 22, 2015 (U.S. Pat. No. 9,850,507), which is aU.S. national stage of International Patent Application No.PCT/US2013/052069, filed Jul. 25, 2013, which claims benefit ofProvisional Application No. 61/675,788, filed Jul. 25, 2012, each ofwhich is incorporated herein by reference.

The instant application contains a sequence listing which has beensubmitted electronically in ASCII format and is hereby incorporated byreference in its entirety. Said ASCII copy, created on Dec. 9, 2017, isnamed “105464_000035_SL.txt” and is 527,808 bytes in size.

This invention relates to recombinant yeast having an active reductiveTCA pathway from pyruvate to succinate. The inventions disclosed andclaimed herein were made pursuant to a joint research agreement betweenCargill Incorporated, Wayzta, Minn., US, and BioAmber S.A.S, Bazancourt,France.

Succinic acid is a chemical intermediate useful as a precursor formaking compounds such as 1,4-butanediol, tetrahydrofuran andgamma-butyrolactone. It is also a useful diacid that can be polymerizedwith a polyol to make polyester resins. Succinic acid can be producedindustrially from butane. However, butane is a petrochemical, and thereis a strong desire to develop processes for making many chemicalcompounds from annually renewable resources such as plant or animalfeedstocks.

Some microorganisms have evolved the ability to produce succinate fromcarbohydrate feedstocks. In some cases, these strains have beenengineered to improve yield and/or productivity. WO 2007/061590describes recombinant yeast cells that produce succinate. Some yeastspecies are of interest as candidates for succinic acid-producingfermentations because they are resistant to low pH conditions, and socan produce acidic fermentation products at a low pH at which theproduct acid exists mainly in the acid form rather than in the saltform. Producing the acid directly in the acid form simplifies recoveryand purification, as salt splitting, with its attendant requirements forraw materials, capital, operating and disposal costs, can be reduced ifnot eliminated.

There are three primary fermentation pathways for by which amicroorganism can produce succinate: oxidative tricarboxylic acid (TCA),glyoxylate shunt, and reductive TCA. The oxidative TCA pathway beginswith the conversion of oxaloacetate (OAA) and acetyl-CoA to citrate. OAAcan be generated from carboxylation of phosphoenolpyruvate (PEP) orpyruvate, while acetyl-CoA is generated from the decarboxylation ofpyruvate by pyruvate dehydrogenase (PDH) or pyruvate formate lyase(PFL). Citrate is converted to isocitrate, isocitrate is converted toα-ketoglutarate, α-ketoglutarate is converted to succinyl-CoA, andsuccinyl-CoA is converted to succinate.

Like the oxidative TCA pathway, the glyoxylate shunt pathway begins withthe generation of citrate from OAA and acetyl-CoA and the conversion ofcitrate to isocitrate. Isocitrate is converted to glyoxylate andsuccinate. Glyoxylate is condensed with acetyl-CoA to form malate, andthe resultant malate is converted to succinate via a fumarateintermediate.

The reductive TCA pathway begins with carboxylation ofphosphoenolpyruvate (PEP) or pyruvate to oxaloacetate (OAA) (by PEPcarboxylase (PPC) and pyruvate carboxylase (PYC), respectively). OAA isconverted to malate by malate dehydrogenase (MDH), malate is convertedto fumarate by fumarase (FUM, also known as fumarate hydratase), andfumarate is converted to succinate by fumarate reductase (FRD). Thereductive TCA pathway provides the highest succinate yield of the threesuccinate fermentation pathways, per mole of glucose consumed, and forthat reason offers the best economic potential.

A problem with the reductive TCA pathway is that the MDH enzyme consumesNADH as a cofactor. In addition, certain efficient FRD enzymes alsoconsume NADH. Examples of such NADH-dependent FRD enzymes are described,for example, in WO 2009/065778 and PCT/US2011/022612. Thus, certainefficient metabolic pathways from pyruvate to succinate consume twomolecules of NADH. One molecule of NADH is produced when sugars such asglucose are metabolized to pyruvate via the glycolytic pathway, but thisstill leaves a net deficit of one NADH, which results in a redoximbalance. A living cell must correct this redox balance if it is toremain healthy and continue to metabolize through the reductive TCApathway. This typically means that the cell must balance the net NADHconsumption by replacing the consumed NADH from other metabolicprocesses that produce NADH. For example, the reductive TCA pathway canbe combined with one or both of the oxidative TCA or glyoxylate shuntpathways to help with the redox balance, but the oxidative TCA andglyoxylate shunt pathways produce less succinic acid per mole ofstarting sugar, and taking this approach therefore results in a loss ofyield. It is possible for the cell to use one or more unrelated pathwaysto produce the needed NADH, but this can have adverse consequences forcell health and productivity, and may create other imbalances within thecell.

Therefore, there remains a desire to provide a yeast strain thatefficiently produces succinic acid (or its salts).

In one aspect, this invention is a recombinant yeast cell having anactive reductive TCA metabolic pathway from pyruvate to succinate andwhich further overexpresses a NAD(P)⁺ transhydrogenase enzyme.

In particular embodiments, the yeast cell of the invention hasintegrated into its genome at least one exogenous NAD(P)+transhydrogenase gene that encodes for the NAD(P)+ transhydrogenaseenzyme.

In other particular embodiments, the recombinant yeast cell of theinvention (a) expresses an NADPH-dependent malate dehydrogenase enzyme,(b) has at least one exogenous NADPH-dependent malate dehydrogenase geneintegrated into its genome, (c) expresses an NADPH-dependent fumaratereductase enzyme, (d) has at least one exogenous NADPH-dependentfumarate reductase gene integrated into its genome or (e) has acombination of any two or more of (a), (b), (c) and (d).

The recombinant yeast cell of the invention in some embodiments hasintegrated into its genome one or more of (i) an exogenous pyruvatecarboxylase gene that encodes for an enzyme which catalyzes theconversion of pyruvate to oxaloacetate, (ii) an exogenous malatedehydrogenase gene which encodes for an enzyme that catalyzes theconversion of oxaloacetate to malate, (iii) an exogenous fumarase genethat encodes for an enzyme which catalyzes the conversion of malate tofumarate and (iv) an exogenous fumarate reductase gene that encodes anenzyme which catalyzes the conversion of fumarate to succinate. In someembodiments, the recombinant cell of the invention has integrated intoits genome one or more of (i) a non-native pyruvate carboxylase genethat encodes for an enzyme which catalyzes the conversion of pyruvate tooxaloacetate, (ii) a non-native malate dehydrogenase gene which encodesfor an enzyme that catalyzes the conversion of oxaloacetate to malate,(iii) a non-native exogenous fumarase gene that encodes for an enzymewhich catalyzes the conversion of malate to fumarate and (iv) anon-native exogenous fumarate reductase gene which encodes an enzymewhich catalyzes the conversion of fumarate to succinate.

In preferred embodiments, the recombinant cell of the invention hasintegrated into its genome at least one exogenous malate dehydrogenasegene which encodes for an NADH-dependent enzyme that catalyzes theconversion of oxaloacetate to malate. In other preferred embodiments,the recombinant cell of the invention has integrated into its genome atleast one exogenous fumarate reductase gene which encodes for anNADH-dependent enzyme that catalyzes the conversion of fumarate tosuccinate. In especially preferred embodiments, the recombinant cell ofthe invention has both of these features.

In other specific embodiments, the recombinant cell of the inventionoverexpresses at least one enzyme which catalyzes a reaction thatincludes the reduction of NADP+ to NADPH. This reaction may be areaction in the pentose phosphate pathway. The enzyme catalyzing thatreaction may be, for example, a 6-phosphogluconate dehydrogenase (6PDGH)enzyme and/or a glucose 6-phosphate dehydrogenase (G6PDH) enzyme.

In still other specific embodiments, the recombinant cell of theinvention overexpresses at least one Stb5p protein, and/or has at leastone exogenous Stb5p gene (i.e., a gene that encodes for the Stb5pprotein) integrated into its genome.

In still other specific embodiments, the recombinant cell of theinvention has a deletion or disruption of a native phosphoglucoseisomerase gene.

In the cells of any of the foregoing aspects of the invention, theNADH/NAD+ redox imbalance that is produced in the reductive TCA pathwayto succinate is compensated for, at least in part, by converting NADPHformed in other cellular metabolic processes to NADH, which can beconsumed in the succinate-producing pathway. This is a beneficialapproach to solving the NADH/NAD+ redox imbalance, because yeast cellstypically have, or can be easily engineered to have, active metabolicpathways that produce NADPH. A yeast cell's native pentose phosphatepathway is an example of a metabolic pathway that produces NADPH. Thus,NADPH can be produced in the cell by directing carbon flux through apentose phosphate pathway, and all or a portion of the NADPH so producedcan be converted to NADH by action of the overexpressed NAD(P)⁺transhydrogenase enzyme. Some or all of the NADH so produced canalleviate or even eliminate the NADH/NAD+ redox imbalance that resultsfrom succinate production through the reductive TCA pathway.

NADPH production can be increased (relative to the wild-type host cell),for example, by increasing carbon flux through the pentose phosphatepathway and/or by overexpressing at least one enzyme (including anenzyme in the pentose phosphate pathway) which catalyzes a reaction thatincludes the reduction of NADP+ to NADPH. Again, the increased NADPH soproduced can be converted to NADH by action of the NAD(P)⁺transhydrogenase enzyme. As before, some or all of the NADH so producedcan alleviate or even eliminate the NADH/NAD+ redox imbalance thatresults from succinate production through the reductive TCA pathway.

This, in some embodiments, the recombinant cell of the inventionincludes one or more genetic modifications that (1) increase fluxthrough the pentose phosphate pathway and/or (2) overexpress one or moreenzymes in the pentose phosphate pathway that catalyze a reaction thatincludes the reduction of NADP+ to NADPH. In certain embodiments,therefore, the recombinant cell of the invention also (a) overexpressesat least one Stb5p protein (b) has at least one exogenous Stb5p geneintegrated into its genome, (c) produces a severely reduced quantity ofan active phosphoglucose isomerase (PGI) enzyme, (d) produces a PGIenzyme that has a severely reduced activity, (e) has a deletion ordisruption of a native PGI gene, (f) overexpresses at least one6-phosphogluconate dehydrogenase (6PGDH) enzyme, (g) has at least oneexogenous 6PGDH gene integrated into its genome, (h) overexpresses atleast one glucose-6-phosphate dehydrogenase (G6PDH) enzyme, (i) has atleast one exogenous G6PDH gene integrated into its genome, or (j) ancombination of any two or more of (a)-(i).

The cell of the invention may produce succinate and transport it fromthe cell. In some embodiments, the cell may further metabolize some orall of the succinate into one or more other succinate metabolizationproducts, and transport one or more of such succinate metabolizationproducts from the cell. In such embodiments, the cell contains native ornon-native metabolic pathways which perform the further metabolizationof succinate into such succinate metabolization product(s).

In yet other aspects, the invention is a method of producing succinateor a metabolization product of succinate, comprising culturing a cell ofany of the foregoing aspects in a fermentation medium that includes atleast one carbon source. The cells of the invention are capable ofproducing succinate or metabolization products of succinate in highyields at commercially reasonable production rates.

The term “NADH-dependent” as used herein refers to the property of anenzyme to preferentially use NADH as the redox cofactor. AnNADH-dependent enzyme has a higher specificity constant (k_(cat)/K_(M))with the cofactor NADH than with other cofactors, including the cofactorNADPH, as determined by in vitro enzyme activity assays.

For purposes of this application, “native” as used herein with regard toa metabolic pathway refers to a metabolic pathway that exists and isactive in the wild-type host strain. Genetic material such as genes,promoters and terminators is “native” for purposes of this applicationif the genetic material has a sequence identical to (apart fromindividual-to-individual mutations which do not affect function) agenetic component that is present in the genome of the wild-type hostcell (i.e., the exogenous genetic component is identical to anendogenous genetic component).

For purposes of this application, genetic material such as a gene, apromoter and a terminator is “endogenous” to a cell if it is (i) nativeto the cell, (ii) present at the same location as that genetic materialis present in the wild-type cell and (iii) under the regulatory controlof its native promoter and its native terminator.

For purposes of this application, genetic material such as genes,promoters and terminators is “exogenous” to a cell if it is (i)non-native to the cell and/or (ii) is native to the cell, but is presentat a location different than where that genetic material is present inthe wild-type cell and/or (iii) is under the regulatory control of anon-native promoter and/or non-native terminator. Extra copies of nativegenetic material are considered as “exogenous” for purposes of thisinvention, even if such extra copies are present at the same locus asthat genetic material is present in the wild-type host strain.

As used herein, the term “promoter” refers to an untranslated sequencelocated upstream (i.e., 5′) to the translation start codon of a gene(generally a sequence of about 1 to 1500 base pairs (bp), preferablyabout 100 to 1000 bp and especially of about 200 to 1000 bp) whichcontrols the start of transcription of the gene. The term “terminator”as used herein refers to an untranslated sequence located downstream(i.e., 3′) to the translation finish codon of a gene (generally asequence of about 1 to 1500 bp, preferably of about 100 to 1000 bp, andespecially of about 200 to 500 bp) which controls the end oftranscription of the gene. A promoter or terminator is “operativelylinked” to a gene if its position in the genome relative to that of thegene is such that the promoter or terminator, as the case may be,performs its transcriptional control function.

“Identity” for nucleotide or amino acid sequences are for purposes ofthis invention calculated using BLAST (National Center for BiologicalInformation (NCBI) Basic Local Alignment Search Tool) version 2.2.13software with default parameters. A sequence having an identity score ofXX % with regard to a reference sequence using the BLAST version 2.2.13algorithm with default parameters is considered to be at least XX %identical or, equivalently, have XX % sequence identity to the referencesequence.

“Deletion or disruption” with regard to a gene means that either theentire coding region of the gene is eliminated (deletion) or the codingregion of the gene, its promoter, and/or its terminator region ismodified (such as by deletion, insertion, or mutation) such that thegene no longer produces an active enzyme, produces a severely reducedquantity (at least 75% reduction, preferably at least 85% reduction,more preferably at least 95% reduction) of the enzyme, or produces anenzyme with severely reduced (at least 75% reduced, preferably at least85% reduced, more preferably at least 95% reduced) activity. A deletionor disruption of a gene can be accomplished by, for example, forcedevolution, mutagenesis or genetic engineering methods, followed byappropriate selection or screening to identify the desired mutants.

“Overexpress” means the artificial expression of an enzyme in increasedquantity by a gene. Overexpression of an enzyme may result from thepresence of one or more exogeneous gene(s), or from other conditions.For purposes of this invention, a yeast cell containing at least oneexogenous gene is considered to overexpress the enzyme(s) encoded bysuch exogenous gene(s).

The recombinant yeast of the invention is made by performing certaingenetic modifications to a host yeast cell. The host yeast cell is onewhich as a wild-type strain is natively capable of metabolizing at leastone sugar to pyruvate. Suitable host yeast cells include (but are notlimited to) yeast cells classified under the genera Candida, Pichia,Saccharomyces, Schizosaccharomyces, Zygosaccharomyces, Kluyveromyces,Debaryomyces, Pichia, Issatchenkia, Yarrowia and Hansenula. Examples ofspecific host yeast cells include C. sonorensis, K. marxianus, K.thermotolerans, C. methanesorbosa, Saccharomyces bulderi (S. bulderi),I. orientalis, C. lambica, C. sorboxylosa, C. zemplinina, C. geochares,P. membranifaciens, Z. kombuchaensis, C. sorbosivorans, C. vanderwaltii,C. sorbophila, Z. bisporus, Z. lentus, Saccharomyces bayanus (S.bayanus), D. castellii, C, boidinii, C. etchellsii, K. lactis, P.jadinii, P. anomala, Saccharomyces cerevisiae (S. cerevisiae) Pichiagaleiformis, Pichia sp. YB-4149 (NRRL designation), Candida ethanolica,P. deserticola, P. membranifaciens, P. fermentans and Saccharomycopsiscrataegensis (S. crataegensis). Suitable strains of K. marxianus and C.sonorensis include those described in WO 00/71738 A1, WO 02/42471 A2, WO03/049525 A2, WO 03/102152 A2 and WO 03/102201A2. Suitable strains of I.orientalis are ATCC strain 32196 and ATCC strain PTA-6648.

In some embodiments of the invention the host cell is Crabtree negativeas a wild-type strain. The Crabtree effect is defined as the occurrenceof fermentative metabolism under aerobic conditions due to theinhibition of oxygen consumption by a microorganism when cultured athigh specific growth rates (long-term effect) or in the presence of highconcentrations of glucose (short-term effect). Crabtree negativephenotypes do not exhibit this effect, and are thus able to consumeoxygen even in the presence of high concentrations of glucose or at highgrowth rates.

In some embodiments, the host cell is succinate-resistant as a wild-typestrain. A cell is considered to be “succinate-resistant” if the cellexhibits a growth rate in media containing 75 g/L or greater succinateat pH 2.8 that is at least 50% as high as its growth rate in the samemedia containing 0 g/L succinate, according to the test method describedin Example 1A of WO 2012/103261.

In some embodiments, the host cell exhibits a volumetric glucoseconsumption rate of at least 3, at least 5 or at least 8 grams ofglucose per liter of broth per hour, as a wild-type strain.

In some embodiments, the host cell exhibits a specific glucoseconsumption rate of at least 0.5, at least 1.0 or at least 1.5 gram ofglucose per gram dry weight of cells per hour, as a wild-type strain.

Volumetric and specific glucose consumption can be measured bycultivating the cells in shake flasks yeast in extract peptone dextrose(YPD) media containing 0 g/l 75 g/L succinate at pH 3.0 a described inExample 1 of WO 2012/103261. The flasks are inoculated with biomassharvested from seed flasks grown overnight to an OD₆₀₀ of 6 to 10. 250mL baffled glycolytic assay flasks (50 mL working volume) are inoculatedto an OD₆₀₀ of 0.1 and grown at 250 RPM and 30° C. Samples are takenthroughout the time course for the assay and analyzed for glucoseconsumption by electrophoretic methods (such as by using a 2700Biochemistry Analyzer from Yellow Springs Instruments or equivalentdevice). The data is plotted and volumetric glucose consumption ratecalculated. Specific glucose consumption rate is calculated by dividingthe glucose consumption by the cell dry weight at the end offermentation.

The genetically modified yeast cells provided herein have an activereductive TCA active pathway from pyruvate to succinate. Such an activereductive TCA pathway includes a step of converting pyruvate orphosphoenolpyruvate (PEP) (or each) to oxaloacetate (OAA), a step ofconverting oxaloacetate to malate, a step of converting malate tofumarate, and a step of converting fumarate to succinate.

The step of converting pyruvate to OAA is catalyzed by a PYC (pyruvatecarboxylase) enzyme, i.e., an enzyme having the ability to catalyze theconversion of pyruvate to OAA. A PYC enzyme is encoded by a PYC(pyruvate carboxylase) gene integrated into the genome of therecombinant yeast cell. The PYC gene may be native or non-native to thehost cell, and may be endogenous (if native) or exogenous (if non-nativeor if additional copies of a native gene are present). In certainembodiments, a PYC gene may be a yeast gene. For example, the PYC genemay be an I. orientalis PYC gene encoding for an enzyme having aminoacid sequence SEQ ID NO: 94, an S. cerevisiae PYC1 gene encoding for anenzyme having amino acid sequence SEQ ID NO: 95, or a K. marxianus PYC1gene encoding for an enzyme having amino acid SEQ ID NO: 96. In otherembodiments, the gene may encode for an enzyme having an amino acidsequence with at least 50%, at least 60%, at least 70%, at least 80%, atleast 85%, at least 90%, at least 95%, at least 97%, or at least 99%sequence identity to any of SEQ ID NOs: 94, 95 or 96. In certainembodiments, the gene may have the nucleotide sequence set forth in SEQID NOs: 4, 45 or 46, or a nucleotide sequence with at least 50%, atleast 60%, at least 70%, at least 80%, at least 85%, at least 90%, atleast 95%, at least 97%, or at least 99% sequence identity to any of SEQID NOs: 4, 45 or 46. In other embodiments, the PYC gene may be fungal.

The step of converting PEP to OAA is catalyzed by a PPC(phosphoenolpyruvate carboxylase) enzyme, i.e., an enzyme having theability to catalyze the conversion of PEP to OAA. A PPC enzyme isencoded by a PPC (phosphoenolpyruvate carboxylase) gene integrated intothe genome of the recombinant yeast cell. The PPC gene may be native ornon-native to the host cell, and may be endogenous (if native) orexogenous (if non-native or if additional copies of a native gene arepresent). The PPC gene may encode for an enzyme having either of aminoacid sequences SEQ ID NO: 97 or 115, or for an enzyme having an aminoacid sequence with at least 50%, at least 60%, at least 70%, at least80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least99% sequence identity to either of SEQ ID NOs: 97 or 115. In certainembodiments, the PPC gene may have the nucleotide sequence set forth ineither of SEQ ID NOs: 49 or 50, or a nucleotide sequence with at least50%, at least 60%, at least 70%, at least 80%, at least 85%, at least90%, at least 95%, at least 97%, or at least 99% sequence identity toeither of SEQ ID NOs: 49 or 50.

The step of converting OAA to malate is catalyzed by a MDH (malatedehydrogenase) enzyme, i.e., an enzyme having the ability to catalyzethe conversion of OAA to malate. A MDH enzyme is encoded by a MDH(malate dehydrogenase) gene present in the genome of the recombinantyeast cell. The MDH gene may be native or non-native to the host cell,and may be endogenous (if native) or exogenous (if non-native or ifadditional copies of a native gene are present). The MDH enzymepreferably is NADH-dependent, i.e., one which uses NADH preferentiallyas a cofactor, and in converting OAA to malate also oxidizes NADH toNAD+. In the cells of this invention, the MDH enzyme preferably isoverexpressed, by integrating one or more copies of an exogenous MDHgene (preferably at least two copies) into the genome of the cell.Preferred MDH genes encode for NADH-dependent MDH enzymes.

In certain embodiments, the MDH gene is a yeast MDH gene that encodesfor an NADH-dependent MDH enzyme. For example, the MDH gene may be an I.orientalis MDH1, MDH2, or MDH3 gene encoding for an enzyme having any ofthe amino acid sequences SEQ ID NOs: 98, 99 or 100, respectively, a Z.rouxii MDH gene encoding for an enzyme having amino acid sequence SEQ IDNO: 101, a K. marxianus MDH1, MDH2, or MDH3 gene encoding for an enzymehaving any of amino acid sequences SEQ ID NOs: 102, 103 or 104,respectively, or a gene encoding for an enzyme having an amino acidsequence with at least 50%, at least 60%, at least 70%, at least 80%, atleast 85%, at least 90%, at least 95%, at least 97%, or at least 99%sequence identity to any thereof. In certain embodiments, the yeast MDHgene has the nucleotide sequence set forth in any of SEQ ID NOs: 58, 59,60, 61, 62, 63 or 64 or a nucleotide sequence with at least 50%, atleast 60%, at least 70%, at least 80%, at least 85%, at least 90%, atleast 95%, at least 97%, or at least 99% sequence identity to anythereof.

In certain embodiments, the MDH gene is a bacterial MDH gene thatencodes for an NADH-dependent MDH enzyme. For example, the MDH gene isin some embodiments an Escherichia coli (E. coli) MDH gene encoding foran enzyme having amino acid sequence SEQ ID NO: 105 or a gene thatencodes for an enzyme having an amino acid sequence with at least 50%,at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, atleast 95%, at least 97%, or at least 99% sequence identity thereto. Incertain embodiments, the bacterial MDH gene has the nucleotide sequenceSEQ ID NO: 66 or a nucleotide sequence with at least 50%, at least 60%,at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, atleast 97%, or at least 99% sequence identity to either of those.

In certain embodiments, an MDH gene is a fungal MDH gene that encodesfor an NADH-dependent MDH enzyme. For example, the MDH gene in someembodiments is a Rhizopus. oryzae (R. oryzae) MDH gene or a Rhizopusdelemar (R. delemar) MDH gene encoding for an enzyme having amino acidsequence SEQ ID NO: 106 or 128 or a gene which encodes for an enzymehaving amino acid sequence with at least 50%, at least 60%, at least70%, at least 80%, at least 85%, at least 90%, at least 95%, at least97%, or at least 99% sequence identity to either thereof. In certainembodiments, the fungal MDH gene has nucleotide sequence SEQ ID NO: 68or 13 or a nucleotide sequence with at least 50%, at least 60%, at least70%, at least 80%, at least 85%, at least 90%, at least 95%, at least97%, or at least 99% sequence identity thereto.

The step of converting malate to fumarate is catalyzed by a FUM(fumarase) enzyme, i.e., an enzyme having the ability to catalyze theconversion of malate to fumarate. A FUM (fumarase) enzyme is encoded bya FUM (fumarase) gene integrated into the genome of the recombinantyeast cell. The FUM gene may be native or non-native to the host cell,and may be endogenous (if native) or exogenous (if non-native or ifadditional copies of a native gene are present). In certain embodiments,a FUM gene is a yeast gene. The FUM gene is in some embodiments an I.orientalis FUM gene encoding an enzyme having amino acid sequence SEQ IDNO: 107, or for an enzyme having an amino acid sequence with at least50%, at least 60%, at least 70%, at least 80%, at least 85%, at least90%, at least 95%, at least 97%, or at least 99% sequence identity toSEQ ID NO: 107. In certain embodiments, the FUM gene may have nucleotidesequence SEQ ID NO: 70 or a nucleotide sequence with at least 50%, atleast 60%, at least 70%, at least 80%, at least 85%, at least 90%, atleast 95%, at least 97%, or at least 99% sequence identity to SEQ ID NO:70. In other embodiments, a FUM gene may be a bacterial gene.

The step of converting fumarate to succinate is catalyzed by a FRD(fumarate reductase) enzyme, i.e., an enzyme having the ability tocatalyze the conversion of fumarate to succinate. A FRD (fumaratereductase) enzyme is encoded by a FRD (fumarate reductase) gene presentin the genome of the recombinant yeast cell. The FRD gene may be nativeor non-native to the host cell, and may be endogenous (if native) orexogenous (if non-native or if additional copies of a native gene arepresent). The FRD enzyme preferably is NADH-dependent, i.e., one whichuses NADH preferentially as a cofactor, and in converting fumarate tosuccinate also oxidizes NADH to NAD+. In the cells of this invention,the FRD enzyme preferably is overexpressed, by integrating one or morecopies of an exogenous FRD gene (preferably at least two copies) intothe genome of the cell. The FRD gene preferably encodes for anNADH-dependent FRD enzyme.

In certain embodiments, the FRD gene is a yeast FRD gene that encodesfor an NADH-dependent FRD enzyme. For example, the FRD gene may be an S.cerevisiae FRD1 gene encoding for an enzyme having amino acid sequenceSEQ ID NO: 108, a Saccharomyces mikatae (S. mikatae) FRD1 gene encodingfor an enzyme having amino acid sequence SEQ ID NO: 109, a K. polysporaFRD1 gene encoding for an enzyme having amino acid sequence SEQ ID NO:110, a K. marxianus FRD1 gene encoding for an enzyme having amino acidsequence SEQ ID NO: 111, or a gene encoding for an enzyme having anamino acid sequence with at least 50%, at least 60%, at least 70%, atleast 80%, at least 85%, at least 90%, at least 95%, at least 97%, or atleast 99% sequence identity to any thereof. In certain embodiments, theyeast FRD gene may have any of nucleotide sequences SEQ ID NOs: 75, 76,77 or 78, or have a nucleotide sequence with at least 50%, at least 60%,at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, atleast 97%, or at least 99% sequence identity to any thereof.

In certain embodiments, the FRD gene may be a protozoan gene thatencodes for an NADH-dependent FRD enzyme. For example, the FRD gene maybe a Trypanosoma brucei (T. brucei) FRD gene encoding for an enzymehaving amino acid sequence SEQ ID NO: 112, a Trypanosoma cruzi (T.cruzi) FRD gene encoding for an enzyme having amino acid sequence SEQ IDNO: 113, a Leishmania braziliensis (L. braziliensis) FRD gene encodingfor an enzyme having amino acid sequence SEQ ID NO: 114, a Leishmaniamexicana (L. mexicana) FRD gene encoding for an enzyme having amino acidsequence SEQ ID NO: 82, or a gene encoding for an enzyme having an aminoacid sequence having at least 50%, at least 60%, at least 70%, at least80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least99% sequence identity to any thereof. In certain embodiments, the FRDgene may have a nucleotide sequence as set forth in any of SEQ ID NOs:42, 43, 44 or 10, or a nucleotide sequence with at least 50%, at least60%, at least 70%, at least 80%, at least 85%, at least 90%, at least95%, at least 97%, or at least 99% sequence identity to any thereof.

In this invention, it is preferred that the reaction of OAA to malateand the reaction of fumarate to succinate each oxidizes NADH to NAD+.The oxidation of NADH to NAD+ typically occurs in cases in which thereaction in any one or more of these steps is catalyzed by anNADH-dependent enzyme as described before.

The recombinant cell of the invention overexpresses an active NAD(P)+transhydrogenase enzyme and/or includes one or more exogenous NAD(P)+transhydrogenase genes, which may be native or non-native to the hostcell. A “NAD(P)+ transhydrogenase” (SthA) gene refers to any gene thatencodes a polypeptide that catalyzes the reaction of NADP(H) to formNAD(H). The NAD(P)+ transhydrogenase (SthA) enzyme preferably is solublein the cytosol of the recombinant cell. The exogenous SthA gene may beof bacterial, fungal, yeast or other origin. The exogenous SthA gene insome embodiments is an E. coli, Azotobacter vinelandii (A. vinelandii)or Pseudomona flourescens SthA gene. The exogenous SthA gene in someembodiments encodes for an enzyme having any of amino acid sequences SEQID NOs: 117, 118, 119, or 146, or which is at least 50%, at least 60%,at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, atleast 97%, or at least 99% identical to any thereof. In certainembodiments, the exogenous SthA gene has any of nucleotide sequences SEQID NOs: 21, 24, 27, or 139, or a nucleotide sequence with at least 50%,at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, atleast 95%, at least 97%, or at least 99% sequence identity to anythereof.

In some embodiments, the recombinant cell exhibits increased flux(relative to the wild-type host strain) through the pentose phosphatepathway and/or overexpresses at least one enzyme which catalyzes areaction that includes the reduction of NADP+ to NADPH.

The overexpressed enzyme may be an enzyme that catalyzes a reaction inthe pentose phosphate pathway. The pentose phosphate pathway metabolizesglucose-6-phosphate to glyceraldehyde-3-phosphate through6-phosphogluconolactone, 6-phosphogluconate and ribulose 5-phosphateintermediates. The conversion of glucose-6-phosphate to6-phosphogluconolactone is catalyzed by a glucose-6-phosphatedehydrogenase (G6PDH) enzyme that uses NADP+ as a cofactor, therebyreducing NADP+ to NADPH. Similarly, the conversion of 6-phosphogluconateto ribulose-5-phosphate is catalyzed by a 6-phosphogluconatedehydrogenase (6PGDH) enzyme that uses NADP+ as a cofactor, therebyreducing NADP+ to NADPH. Overexpessing one or both of these enzymes, orincreasing flux through the pentose phosphate pathway, produces NADPH,which can be converted to NADH by action of the NAD(P)+ transhydrogenaseenzyme, helping to maintain cofactor balance in the cell.

One way of increasing flux through the pentose phosphate pathway is todisrupt the glycolytic pathway from glucose to pyruvate. This can bedone, for example, by disrupting or removing the step of isomerisingglucose-6-phosphate to fructose-6-phosphate, which is catalyzed by aphosphoglucose (PGI) enzyme. Therefore, in certain embodiments, therecombinant cell of the invention produces a severely reduced quantity(at least 75% reduction, preferably at least 85% reduction, morepreferably at least 95% reduction) of an active phosphoglucose isomerase(PGI) enzyme, or produces a PGI enzyme with severely reduced (at least75% reduced, preferably at least 85% reduced, more preferably at least95% reduced) activity. In some embodiments, the recombinant cellincludes a deletion or disruption of at least one native phosphoglucoseisomerase (PGI) gene. If the host cell contains multiple alleles of thePGI gene, all such alleles may be deleted or disrupted.

The overexpressed enzyme which catalyzes a reaction that includes thereduction of NADP+ to NADPH may be an enzyme that catalyzes a reactionin the pentose phosphate pathway. The pentose phosphate pathwaymetabolizes glucose-6-phosphate to glyceraldehyde-3-phosphate through6-phosphogluconolactone, 6-phosphogluconate and ribulose 5-phosphateintermediates. The conversion of glucose-6-phosphate to6-phosphogluconolactone is catalyzed by a glucose-6-phosphatedehydrogenase (G6PDH) enzyme that uses NADP+ as a cofactor, therebyreducing NADP+ to NADPH. Similarly, the conversion of 6-phosphogluconateto ribulose-5-phosphate is catalyzed by a 6-phosphogluconatedehydrogenase (6PGDH) enzyme that uses NADP+ as a cofactor, therebyreducing NADP+ to NADPH.

Therefore, in certain embodiments, the yeast cell of the inventionoverexpresses a G6PDH enzyme. Such a yeast cell in some embodimentsincludes one or more exogenous G6PDH genes, which may be native ornon-native to the strain, integrated into its genome. In certain ofthese embodiments, the exogenous G6PDH gene may be an I. orientalisG6PDH gene (ZWF1) that encodes for an enzyme having amino acid sequenceSEQ ID NO: 121 or which encodes for an enzyme having with at least 50%,at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, atleast 95%, at least 97%, or at least 99% sequence identity SEQ ID NO:121. In certain embodiments, the G6PDH gene may have nucleotide sequenceSEQ ID NO: 87 or a nucleotide sequence with at least 50%, at least 60%,at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, atleast 97%, or at least 99% sequence identity to nucleotide sequence SEQID NO: 87.

Similarly, in other embodiments, the recombinant yeast cells providedherein contains one or more exogenous 6PGDH genes, which may be nativeor non-native to the host strain, integrated into its genome. In certainembodiments, a 6PGDH gene may be a yeast 6PGDH gene such as an I.orientalis 6PGDH gene. In certain embodiments, the exogenous 6PGDH geneencodes for an enzyme having amino acid sequence SEQ ID NO: 88, or anamino acid sequence with at least 50%, at least 60%, at least 70%, atleast 80%, at least 85%, at least 90%, at least 95%, at least 97%, or atleast 99% sequence identity to SEQ ID NO: 88. In certain embodiments,the exogenous 6PGDH gene has the nucleotide sequence of SEQ ID NO: 89,or a nucleotide sequence with at least 50%, at least 60%, at least 70%,at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, orat least 99% sequence identity to the nucleotide sequence of SEQ ID NO:89.

In certain embodiments, the recombinant cell of the inventionoverexpresses an oxidative stress-activated zinc cluster protein Stb5p.This zinc cluster protein regulates genes involved in certainNADPH-producing reactions, including the G6PDH and 6PGDH genes. Incertain embodiments, the recombinant cell includes one or more exogenousStb5p genes, which may be native or non-native to the host cell,integrated into its genome. In certain embodiments, the exogenous Stb5pgene encodes for an enzyme having amino acid sequence SEQ ID NO: 83, oran amino acid sequence with at least 50%, at least 60%, at least 70%, atleast 80%, at least 85%, at least 90%, at least 95%, at least 97%, or atleast 99% sequence identity to SEQ ID NO: 83. In certain embodiments,the exogenous Stb5p gene has the nucleotide sequence of SEQ ID NO: 30,or a nucleotide sequence with at least 50%, at least 60%, at least 70%,at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, orat least 99% sequence identity to the nucleotide sequence of SEQ ID NO:30.

The recombinant cell of the invention may further include one or moreexogenous succinate exporter genes, which may be native or non-native tothe host cell. A “succinate exporter gene” as used herein refers to anygene that encodes a polypeptide with succinate export activity, meaningthe ability to transport succinate out of a cell and into theextracellular environment. The exogenous succinate exporter gene may bea fungal succinate exporter gene such as a Schizosaccharomyces pombe (S.pombe) succinate exporter gene or Aspergillus oryzae (A. oryzae) sourcesuccinate exporter gene. The exogenous succinate exporter gene in someembodiments encodes for an enzyme having amino acid sequence SEQ ID NOs:90 or 91, or at least 50%, at least 60%, at least 70%, at least 80%, atleast 85%, at least 90%, at least 95%, at least 97%, or at least 99%sequence identity to either of SEQ ID NOs: 90 or 91. In certainembodiments, the exogenous succinate exporter gene has either ofnucleotide sequence SEQ ID NOs: 92 or 93, or a nucleotide sequence withat least 50%, at least 60%, at least 70%, at least 80%, at least 85%, atleast 90%, at least 95%, at least 97%, or at least 99% sequence identityto either SEQ ID NOs: 92 or 93.

In certain embodiments, the recombinant yeast cells provided herein mayhave a deletion or disruption of one or more other endogenous genes. Theother deleted or disrupted genes may include genes which produce enzymesthat catalyze the reaction of pyruvate or phosphoenolpyruvate (or theirmetabolizes) to downstream products other than succinate. Among suchgenes are, for example, native pyruvate decarboxylase, alcoholdehydrogenase 1 (ADH1, catalyzes the conversion of acetaldehyde toethanol), alcohol dehydrogenase 2 (ADH2, catalyzes the conversion ofethanol to acetaldehyde), glycerol-3-phosphate dehydrogenase (GPD,systematic name sn-glycerol-3-phosphate:NAD+2-oxidoreductase, EC1.1.1.8), and glycerol-3-phosphatase enzyme (GPP, systematic nameglycerol-1-phosphate phosphohydrolase, EC 3.1.3.21) and NADH⁺-dependentglycerol dehydrogenase (systematic name glycerol:NAD+2-oxidoreductase,EC 1.1.1.6) genes.

Other endogenous genes that may be deleted in certain embodiments of theinvention include genes which encode for enzymes that catalyze areaction that consumes PEP, pyruvate, succinate or any intermediatesproduced in the reductive TCA pathway (other than the TCA pathwayreactions leading to succinate). Examples of such genes include a nativepyruvate carboxylase gene (which encodes an enzyme that converts OAA topyruvate), a native PEP carboxykinase (PCK) gene (which encodes anenzyme that converts OAA to PEP), a native malic enzyme (MAE) gene(which encodes an enzyme that converts malate to pyruvate) and a nativesuccinate dehydrogenase (SDH) gene (which encodes an enzyme thatcatalyzes the back-reaction of succinate to fumarate).

In some embodiments, the modified yeast cells provided herein have adeletion or disruption of a native succinate importer gene, which asused herein refers to any gene that encodes a polypeptide that allowsfor growth on and consumption of succinate.

In certain embodiments, the cells may contain all or part of an activeoxidative TCA or glyoxylate shunt succinate fermentation pathway. Inthese embodiments, the cells comprise one or more genes encoding enzymesselected from the group consisting of citrate synthase, PDH (pyruvatedehydrogenase), PFL (pyruvate formate lyase), aconitase, IDH (isocitratedehydrogenase), α-KGDH (α-ketoglutarate dehydrogenase), succinatethiokinase, isocitrate lyase, and malate synthase.

The recombinant cell of the invention may further include one or moremodifications which individually or collectively confers to the cell theability to ferment pentose sugars to xylulose 5-phosphate. Among suchmodifications are (1) insertion of a functional xylose isomerase gene,(2) a deletion or disruption of a native gene that produces an enzymethat catalyzes the conversion of xylose to xylitol, (3) a deletion ordisruption of a functional xylitol dehydrogenase gene and/or (4)modifications that cause the cell to overexpress a functionalxylulokinase. Methods for introducing those modifications into yeastcells are described, for example, in WO 04/099381, incorporated hereinby reference. Suitable methods for inserting a functional xyloseisomerase gene, deleting or disrupting a native gene that produces anenzyme that catalyzes the conversion of xylose to xylitol, deleting ordisrupting a functional xylitol dehydrogenase gene, and modifying thecell to overexpress a functional xylulokinase are described, forexample, in WO 04/099381, incorporated herein by reference.

In this invention, any exogenous gene, including without limitation anyof the exogeneous genes in the reductive TCA pathway from pyruvate tosuccinate, any succinate exporter gene, any G6PDH gene, any 6PGDH gene,any SthA gene, or any other exogenous gene introduced into the hostcell, is operatively linked to one or more regulatory elements, and inparticular to a promoter sequence and a terminator sequence that eachare functional in the host cell. Such regulatory elements may be nativeor non-native to the host cell.

Examples of promoters that may be linked to one or more exogenous genesin the yeast cells provided herein include, but are not limited to,promoters for pyruvate decarboxylase (PDC1), phosphoglycerate kinase(PGK), xylose reductase (XR), xylitol dehydrogenase (XDH),L-(+)-lactate-cytochrome c oxidoreductase (CYB2), translation elongationfactor-1 or -2 (TEF1, TEF2), enolase (ENO1), glyceraldehyde-3-phosphatedehydrogenase (GAPDH), orotidine 5′-phosphate decarboxylase (URA3)genes, as well as any of those described in the various Examples thatfollow. Where the promoters are non-native, they may be identical to orshare a high degree of sequence identity (i.e., at least about 80%, atleast about 85%, at least about 90%, at least about 95%, or at leastabout 99%) with one or more native promoters. Other suitable promotersand terminators include those described, for example, in WO99/14335,WO00/71738, WO02/42471, WO003/102201, WO03/102152 and WO03/049525.

Examples of terminators that may be linked to one or more exogenousgenes in the yeast cells provided herein include, but are not limitedto, terminators for PDC1, XR, XDH, transaldolase (TAL), transketolase(TKL), ribose 5-phosphate ketol-isomerase (RKI), CYB2, oriso-2-cytochrome c (CYC) genes or the galactose family of genes(especially the GAL10 terminator), as well as any of those described inthe various Examples that follow. Where the terminators are non-native,they may be identical to or share a high degree of sequence identity(i.e., at least about 80%, at least about 85%, at least about 90%, atleast about 95%, or at least about 99%) with one or more nativeterminators.

Modifications (insertion, deletions and/or disruptions) to the genome ofthe host cell described herein can be performed using methods known inthe art. Exogeneous genes may be integrated into the genome in atargeted or a random manner using, for example, well knownelectroporosis and chemical methods (including calcium chloride and/orlithium acetate methods). In those embodiments where an exogenous geneis integrated in a targeted manner, it may be integrated into the locusfor a particular native gene, such that integration of the exogenousgene is coupled with deletion or disruption of a native gene.Alternatively, the exogenous gene may be integrated into a portion ofthe native genome that does not correspond to a gene. Methods fortransforming a yeast cell with an exogenous construct are described in,for example, WO99/14335, WO00/71738, WO02/42471, WO03/102201,WO003/102152, WO03/049525, WO2007/061590, WO 2009/065778 andPCT/US2011/022612.

Insertion of exogenous genes is generally performed by transforming thecell with one or more integration constructs or fragments. The terms“construct” and “fragment” are used interchangeably herein to refer to aDNA sequence that is used to transform a cell. The construct or fragmentmay be, for example, a circular plasmid or vector, a portion of acircular plasmid or vector (such as a restriction enzyme digestionproduct), a linearized plasmid or vector, or a PCR product preparedusing a plasmid or genomic DNA as a template. An integration constructcan be assembled using two cloned target DNA sequences from an insertionsite target. The two target DNA sequences may be contiguous ornon-contiguous in the native host genome. In this context,“non-contiguous” means that the DNA sequences are not immediatelyadjacent to one another in the native genome, but are instead areseparated by a region that is to be deleted. “Contiguous” sequences asused herein are directly adjacent to one another in the native genome.Where targeted integration is to be coupled with deletion or disruptionof a target gene, the integration construct also functions as a deletionconstruct. In such an integration/deletion construct, one of the targetsequences may include a region 5′ to the promoter of the target gene,all or a portion of the promoter region, all or a portion of the targetgene coding sequence, or some combination thereof. The other targetsequence may include a region 3′ to the terminator of the target gene,all or a portion of the terminator region, and/or all or a portion ofthe target gene coding sequence. Where targeted integration is not to becoupled to deletion or disruption of a native gene, the target sequencesare selected such that insertion of an intervening sequence will notdisrupt native gene expression. An integration or deletion construct isprepared such that the two target sequences are oriented in the samedirection in relation to one another as they natively appear in thegenome of the host cell. The gene expression cassette is cloned into theconstruct between the two target gene sequences to allow for expressionof the exogenous gene. The gene expression cassette contains theexogenous gene, and may further include one or more regulatory sequencessuch as promoters or terminators operatively linked to the exogenousgene.

It is usually desirable that the deletion construct may also include afunctional selection marker cassette. When a single deletion constructis used, the marker cassette resides on the vector downstream (i.e., inthe 3′ direction) of the 5′ sequence from the target locus and upstream(i.e., in the 5′ direction) of the 3′ sequence from the target locus.Successful transformants will contain the selection marker cassette,which imparts to the successfully transformed cell some characteristicthat provides a basis for selection.

A “selection marker gene” may encode for a protein needed for thesurvival and/or growth of the transformed cell in a selective culturemedium. Typical selection marker genes encode proteins that (a) conferresistance to antibiotics or other toxins, (such as, for example, zeocin(Streptoalloteichus hindustanus ble bleomycin resistance gene), G418(kanamycin-resistance gene of Tn903) or hygromycin (aminoglycosideantibiotic resistance gene from E. coli)), (b) complement auxotrophicdeficiencies of the cell (such as, for example, amino acid leucinedeficiency (K. marxianus LEU2 gene) or uracil deficiency (e.g., K.marxianus or S. cerevisiae URA3 gene)); (c) enable the cell tosynthesize critical nutrients not available from simple media, or (d)confer ability for the cell to grow on a particular carbon source, (suchas a MEL5 gene from S. cerevisiae, which encodes the alpha-galactosidase(melibiase) enzyme and confers the ability to grow on melibiose as thesole carbon source). Preferred selection markers include the zeocinresistance gene, G418 resistance gene, a MEL5 gene, a URA3 gene andhygromycin resistance gene. Another preferred selection marker is anL-lactate:ferricytochrome c oxidoreductase (CYB2) gene cassette,provided that the host cell either natively lacks such a gene or thatits native CYB2 gene(s) are first deleted or disrupted.

The construct may be designed so that the selection marker cassette canbecome spontaneously deleted as a result of a subsequent homologousrecombination event. A convenient way of accomplishing this is to designthe vector such that the selection marker gene cassette is flanked bydirect repeat sequences. Direct repeat sequences are identical DNAsequences, native or not native to the host cell, and oriented on theconstruct in the same direction with respect to each other. The directrepeat sequences are advantageously about 50-1500 bp in length. It isnot necessary that the direct repeat sequences encode for anything. Thisconstruct permits a homologous recombination event to occur. This eventoccurs with some low frequency, resulting in cells containing a deletionof the selection marker gene and one of the direct repeat sequences. Itmay be necessary to grow transformants for several rounds onnonselective or selective media to allow for the spontaneous homologousrecombination to occur in some of the cells. Cells in which theselection marker gene has become spontaneously deleted can be selectedor screened on the basis of their loss of the selection characteristicimparted by the selection marker gene, or by using PCR or SouthernAnalysis methods to confirm the loss of the selection marker.

In some embodiments, an exogenous gene may be inserted using DNA fromtwo or more integration fragments, rather than a single fragment. Inthese embodiments, the 3′ end of one integration fragment contains aregion of homology with the 5′ end of another integration fragment. Oneof the fragments will contain a first region of homology to the targetlocus and the other fragment will contain a second region of homology tothe target locus. The gene cassette to be inserted can reside on eitherfragment, or be divided among the fragments, with a region of homologyat the 3′ and 5′ ends of the respective fragments, so the entire,functional gene cassette is produced upon a crossover event. The cell istransformed with these fragments simultaneously. A selection marker mayreside on any one of the fragments or may be divided between thefragments with a region of homology as described. In other embodiments,transformation from three or more constructs can be used in an analogousway to integrate exogenous genetic material.

Deletions and/or disruptions of native genes can be performed bytransformation methods, by mutagenesis and/or by forced evolutionmethods. In mutagenesis methods cells are exposed to ultravioletradiation or a mutagenic substance, under conditions sufficient toachieve a high kill rate (60-99.9%, preferably 90-99.9%) of the cells.Surviving cells are then plated and selected or screened for cellshaving the deleted or disrupted metabolic activity. Disruption ordeletion of the desired native gene(s) can be confirmed through PCR orSouthern analysis methods.

Cells of the invention can be cultivated to produce succinic acid,either in the free acid form or in salt form (or both), or ametabolization product of succinate. The recombinant cell is cultured ina medium that includes at least one carbon source that can be fermentedby the cell. Examples include, but are not limited to, twelve carbonsugars such as sucrose, hexose sugars such as glucose or fructose,glycan, starch, or other polymer of glucose, glucose oligomers such asmaltose, maltotriose and isomaltotriose, panose, and fructose oligomers,and pentose sugars such as xylose, xylan, other oligomers of xylose, orarabinose.

The medium will typically contain, in addition to the carbon source,nutrients as required by the particular cell, including a source ofnitrogen (such as amino acids, proteins, inorganic nitrogen sources suchas ammonia or ammonium salts, and the like), and various vitamins,minerals and the like. In some embodiments, the cells of the inventioncan be cultured in a chemically defined medium.

Other cultivation conditions, such as temperature, cell density,selection of substrate(s), selection of nutrients, and the like are notconsidered to be critical to the invention and are generally selected toprovide an economical process. Temperatures during each of the growthphase and the production phase may range from above the freezingtemperature of the medium to about 50° C., although this depends to someextent on the ability of the strain to tolerate elevated temperatures. Apreferred temperature, particularly during the production phase, isabout 30 to 45° C.

During cultivation, aeration and agitation conditions may be selected toproduce a desired oxygen uptake rate. The cultivation may be conductedaerobically, microaerobically, or anaerobically, depending on pathwayrequirements. In some embodiments, the cultivation conditions areselected to produce an oxygen uptake rate of around 2-25 mmol/L/hr,preferably from around 5-20 mmol/L/hr, and more preferably from around8-15 mmol/L/hr. “Oxygen uptake rate” or “OUR” as used herein refers tothe volumetric rate at which oxygen is consumed during the fermentation.Inlet and outlet oxygen concentrations can be measured with exhaust gasanalysis, for example by mass spectrometers. OUR can be calculated usingthe Direct Method described in Bioreaction Engineering Principles 2ndEdition, 2003, Kluwer Academic/Plenum Publishers, p. 449, equation 1.

The culturing process may be divided up into phases. For example, thecell culture process may be divided into a cultivation phase, aproduction phase, and a recovery phase.

The pH may be allowed to range freely during cultivation, or may bebuffered if necessary to prevent the pH from falling below or risingabove predetermined levels. For example, the medium may be buffered toprevent the pH of the solution from falling below around 2.0 or aboveabout 8.0 during cultivation. In certain of these embodiments, themedium may be buffered to prevent the pH of the solution from fallingbelow around 3.0 or rising above around 7.0, and in certain of theseembodiments the medium may be buffered to prevent the pH of the solutionfrom falling below around 3.0 or rising above around 4.5. Suitablebuffering agents include basic materials that neutralize the acid as itis formed, and include, for example, calcium hydroxide, calciumcarbonate, sodium hydroxide, potassium hydroxide, potassium carbonate,sodium carbonate, ammonium carbonate, ammonia, ammonium hydroxide andthe like.

In a buffered fermentation, acidic fermentation products are neutralizedto the corresponding salt as they are formed. Recovery of the acidtherefore involves regenerating the free acid. This is typically done byremoving the cells and acidulating the fermentation broth with a strongacid such as sulfuric acid. A salt by-product is formed (gypsum in thecase where a calcium salt is the neutralizing agent and sulfuric acid isthe acidulating agent), which is separated from the broth.

In other embodiments, the pH of the fermentation medium may be permittedto drop during cultivation from a starting pH that is at or above thelower pKa (4.207) of succinate, typically 8 or higher, to at or belowthe lower pKa of the acid fermentation product, such as in the range ofabout 2.0 to about 4.2, in the range of from about 3.0 to about 4.2, orin the range from about 3.8 to about 4.2.

In still other embodiments, fermentation may be carried out to produce aproduct acid by adjusting the pH of the fermentation broth to at orbelow the lower pKa of the product acid prior to or at the start of thefermentation process. The pH may thereafter be maintained at or belowthe lower pKa of the product acid throughout the cultivation. In certainembodiments, the pH may be maintained at a range of about 2.0 to about4.2, in the range of from about 3.0 to about 4.2, or in the range fromabout 3.8 to about 4.2.

When the pH of the fermentation broth is low enough that the succinateis present in acid form, the acid can be recovered from the broththrough techniques such as liquid-liquid extraction, distillation,absorption, etc., such as are described in T. B. Vickroy, Vol. 3,Chapter 38 of Comprehensive Biotechnology, (ed. M. Moo-Young), Pergamon,Oxford, 1985; R. Datta, et al., FEMS Microbiol. Rev., 1995, 16:221-231;U.S. Pat. Nos. 4,275,234, 4,771,001, 5,132,456, 5,420,304, 5,510,526,5,641,406, and 5,831,122, and WO 93/00440.

The cultivation may be continued until a yield of succinate on thecarbon source is, for example, at least 10%, at least 20%, at least 30%,at least 40%, at least 50%, or greater than 50% of the theoreticalyield. The yield to succinate may at least 80% or at least 90% of thetheoretical yield. The concentration, or titer, of succinate produced inthe cultivation will be a function of the yield as well as the startingconcentration of the carbon source. In certain embodiments, the titermay reach at least 1, at least 3, at least 5, at least 10, at least 20,at least 30, at least 40, at least 50, or greater than 50 g/L at somepoint during the fermentation, and preferably at the end of thefermentation.

In certain embodiments, the genetically modified yeast cells produceethanol in a yield of 10% or less, preferably in a yield of 2% or lessof the theoretical yield. In certain of these embodiments, ethanol isnot detectably produced. In other embodiments, however, succinate andethanol may be co-produced. In these embodiments, ethanol may beproduced at a yield of greater than 10%, greater than 25%, or greaterthan 50% of the theoretical yield.

The recombinant cell of the invention may exhibit a volumetric glucoseconsumption rate of at least 0.5 gram, at least 0.75 gram, or at least0.9 gram of glucose per liter of broth per hour, when cultivated underthe conditions described in Examples 253-255.

The cell of the invention may produce succinate as an end-product of thefermentation process. In such a case, the cell preferably transportssuccinate out of the cell and into the surrounding culture medium.

In some embodiments, the cell may further metabolize some or all of thesuccinate into one or more succinate metabolization products, i.e., acompound formed in the further metabolization of succinate by the cell.Examples of such downstream succinate metabolization products include,for example, 1,4-butanediol, 1,3-butadiene, propionic acid, and3-hydroxyisobutryic acid. In such embodiments, the cell contains nativeor non-native metabolic pathways which perform the such a furthermetabolization of succinate into such downstream succinatemetabolization product(s). The cell may then transport such downstreamsuccinate metabolization products out of the cell and into thesurrounding medium. In some embodiments, the cell may transport one ormore succinate metabolization products, but not succinate, out of thecell. In other embodiments, the cell may transport both succinate itselfand one or more succinate metabolization products out of the cell. Forexample, the cell may transport less than 10% by weight of succinatefrom the cell, based on the combined weight of succinate and succinatemetabolization products exported from the cell.

The following examples are provided to illustrate the invention, but arenot intended to limit the scope thereof. All parts and percentages areby weight unless otherwise indicated.

EXAMPLES Construction of Preparatory Examples

P-1. An I. orientalis strain host strain is generated by evolving I.orientalis strain ATCC PTA-6658 for 91 days in a glucose-limitedchemostat. The system is fed 15 g/L glucose in a defined medium andoperated at a dilution rate of 0.06 h⁻¹ at pH=3 with added lactic acidin the feed medium. The conditions are maintained with an oxygentransfer rate of approximately 2 mmol L⁻¹h⁻¹, and dissolved oxygenconcentration remains constant at 0% of air saturation. Single colonyisolates from the final time point are characterized in two shake flaskassays. In the first assay, the isolates are characterized for theirability to ferment glucose to ethanol in the presence of 25 g/L totallactic acid with no pH adjustment in the defined medium. In the secondassay, the growth rate of the isolates is measured in the presence of 45g/L of total lactic acid, with no pH adjustment in the defined medium.Strain P-1 is a single isolate exhibiting the highest glucoseconsumption rate in the first assay and the highest growth rate in thesecond assay.

P-2. Strain P-1 is transformed with linearized integration fragment P2(having nucleotide sequence SEQ ID NO: 1) designed to disrupt the URA3gene, using a LiOAc transformation method as described by Gietz et al.,in Met. Enzymol. 350:87 (2002). Integration fragment P2 includes a MEL5selection marker gene. Transformants are selected on YNB-melibioseplates and screened by PCR to confirm the integration of the integrationpiece and deletion of a copy of the URA3 gene. A URA3-deletant strain isgrown for several rounds until PCR screening identifies an isolate inwhich the MEL5 selection marker gene has looped out. The PCR screeningis performed using primers having nucleotide sequences SEQ ID NOs: 47and 48 to confirm the 5′-crossover and primers having nucleotidesequences SEQ ID NOs: 51 and 52 to confirm the 3′ crossover. Thatisolate is again grown for several rounds on 5-fluoroorotic acid (FOA)plates to identify a strain in which the URA3 marker has looped out. PCRscreening is performed on this strain using primers having nucleotidesequences SEQ ID NOs: 56 and 124, identifies an isolate in which bothURA3 alleles have been deleted. This isolate is named strain P-2.

P-3. Strain P-2 is transformed with integration fragment P3 (having thenucleotide sequence SEQ ID NO: 2), which is designed to disrupt the PDCgene. Integration fragment P3 contains the following elements, 5′ to 3′:a DNA fragment with homology for integration corresponding to the regionimmediately upstream of the I. orientalis PDC open reading frame, a PDCtranscriptional terminator, the URA3 promoter, the I. orientalis URA3gene, an additional URA3 promoter direct repeat for marker recycling anda DNA fragment with homology for integration corresponding to the regiondirectly downstream of the I. orientalis PDC open reading frame. Asuccessful integrant (and single-copy PDC deletant) is identified onselection plates lacking uracil and confirmed by PCR using primershaving nucleotide sequences SEQ ID NOS: 53 and 54 to confirm the5′-crossover and primers having nucleotide sequences SEQ ID NOs: 55 and122 to confirm the 3′-crossover. That integrant is grown for severalrounds and plated on 5-fluoroorotic acid (FOA) plates to identify astrain in which the URA3 marker has looped out. Loopout of the URA3marker is confirmed by PCR. That strain is again transformed withintegration fragment P3 (SEQ ID NO: 2) to delete the second copy of thenative PDC gene. A successful transformant is again identified byselection on selection plates lacking uracil, and further confirmed byculturing the strain over two days and measuring ethanol production.Lack of ethanol production further demonstrates a successful deletion ofboth copies of the PDC gene in a transformant. That transformant isgrown for several rounds and plated on FOA plates until PCR identifies astrain in which the URA3 marker has looped out. The PCR screening isperformed using primers having nucleotide sequences SEQ ID NOs: 53 and54 to confirm the 5′-crossover and SEQ ID NOs: 55 and 122 to confirm the3′-crossover. That strain is plated on selection plates lacking uracilto confirm the loss of the URA3 marker, and is designated strain P-3.

P-4. Integration fragment P4-1, having nucleotide sequence SEQ ID NO: 3,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis ADH9091 open reading frame, an I. orientalis PDC1promoter, the I. orientalis PYC gene (having the nucleotide sequence SEQID NO: 4), the I. orientalis TAL terminator, the I. orientalis URA3promoter, and the first 530 bp of the I. orientalis URA3 open readingframe.

Integration fragment P4-2, having nucleotide sequence SEQ ID NO: 5,contains the following elements, 5′ to 3′: a DNA fragment correspondingto the last 568 bp of the I. orientalis URA3 open reading frame, the I.orientalis URA3 terminator, the I. orientalis URA3 promoter, the I.orientalis TDH3 promoter, the S. pombe MAE gene (having the nucleotidesequence SEQ ID NO: 6), the I. orientalis TKL terminator, and a DNAfragment with homology for integration corresponding to the regionimmediately downstream of the I. orientalis ADH9091 open reading frame.

Strain P-3 is transformed simultaneously with integration fragments P4-1and P4-2, using lithium acetate methods, to insert the I. orientalis PYCgene and the S. pombe MAE gene at the ADH9091 locus. Integration occursvia three cross-over events: in the regions of the ADH9091 upstreamhomology, in the regions of the ADH9091 downstream homology and in theregion of URA3 homology between SEQ ID NO: 3 and SEQ ID NO: 5.Transformants are streaked to isolates and the correct integration ofthe cassette at the AHD9091 locus is confirmed in a strain by PCR. ThePCR screening is performed using primers having nucleotide sequences SEQID NOs: 65 and 69 to confirm the 5′-crossover and SEQ ID NOs: 67 and 71to confirm the 3′-crossover. That strain is grown and plated on FOA asbefore until the loopout of the URA3 marker from an isolate is confirmedby PCR.

That isolate is then transformed simultaneously with integrationfragments P4-3 and P4-4 using LiOAc transformation methods, to insert asecond copy of each of the I. orientalis PYC gene and the S. pombe MAEgene at the ADH9091 locus.

Integration fragment P4-3, having the nucleotide sequence SEQ ID NO: 7,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately downstream ofthe I. orientalis ADH9091 open reading frame, an I. orientalis PDC1promoter, the I. orientalis PYC gene as found in SEQ ID NO: 4, the I.orientalis TAL terminator, the I. orientalis URA3 promoter, and thefirst 530 bp of the I. orientalis URA3 open reading frame.

Integration fragment P4-4, having the nucleotide sequence SEQ ID NO: 8,contains the following elements, 5′ to 3′: a DNA fragment correspondingto the last 568 bp of the I. orientalis URA3 open reading frame, the I.orientalis URA3 terminator, the I. orientalis URA3 promoter, the I.orientalis TDH3 promoter, the S. pombe MAE gene (having a nucleotidesequences SEQ ID NO: 6), the I. orientalis TKL terminator, and a DNAfragment with homology for integration corresponding to the regionimmediately upstream of the I. orientalis ADH9091 open reading frame.

Integration again occurs via three crossover events. Transformants arestreaked to isolates and screened by PCR to identify a strain containingboth copies of the I. orientalis PYCI and S. pombe MAE genes at theADH9091 locus by PCR. The PCR screening to confirm the first copy isperformed using primers having nucleotide sequences SEQ ID NOs: 65 and69 to confirm the 5′-crossover and SEQ ID NOs: 67 and 71 to confirm the3′-crossover. The PCR screening to confirm the second copy is performedusing primers having nucleotide sequences SEQ ID NOs: 65 and 67 toconfirm the 5′-crossover and SEQ ID NOs: 69 and 71 to confirm the3′-crossover. That strain is grown and replated on FOA until a strain inwhich the URA3 marker has looped out is identified. That strain isdesignated strain P-4.

P-5. Strain P-4 is transformed with integration fragment P5-1 (havingthe nucleotide sequence SEQ ID NO: 9) using LiOAc transformation methodsas described in previous examples, to integrate the L. mexicana FRD geneat the locus of the native CYB2b open reading frame. The integrationfragment P5-1 contains the following elements, 5′ to 3′: a DNA fragmentwith homology for integration corresponding to the region immediatelydownstream of the I. orientalis CYB2b open reading frame, an I.orientalis PDC1 promoter, the L. mexicana FRD gene (having nucleotidesequence SEQ ID NO: 10, and encoding for an enzyme having amino acidsequence SEQ ID NO: 82), the I. orientalis PDC1 Terminator, the I.orientalis URA3 promoter, gene, and terminator in succession, followedby an additional URA3 promoter which serves as a direct repeat formarker recycling, and a region immediately upstream of the I. orientalisCYB2b open reading frame.

Successful integration of a single copy of the L. mexicana FRD gene inone isolate is identified by selection on a selection plates lackinguracil and confirmed by PCR. The PCR screening is performed usingprimers having nucleotide sequences SEQ ID NOs: 72 and 73 to confirm the5′-crossover and SEQ ID NOs: 69 and 79 to confirm the 3′-crossover. Thatisolate is grown and plated on FOA as before until a strain in which theURA3 promoter has looped out is identified by PCR. That isolate istransformed with integration fragment P5-2 in the same manner as before,to integrate a second copy of the L. mexicana FRD gene at the nativelocus of the CYB2b open reading frame.

Integration fragment P5-2 (having nucleotide sequence SEQ ID NO: 11),contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis CYB2b open reading frame, an I. orientalis PDC1 promoter,the L. mexicana FRD gene (having the nucleotide sequence SEQ ID NO: 10),the I. orientalis PDC1 terminator, the I. orientalis URA3 promoter,gene, and terminator in succession, followed by an additional URA3promoter which serves as a direct repeat for marker recycling, and aregion immediately downstream of the I. orientalis CYB2b open readingframe.

Correct integration of the second copy of the L. mexicana FRD gene inone isolate is confirmed by PCR using primers having nucleotidesequences SEQ ID NOs: 69 and 73 to confirm the 5′-crossover and SEQ IDNOs: 72 and 79 to confirm the 3′-crossover. Retention of the firstintegration is reconfirmed by repeating the PCR reactions used to verifyproper integration of fragment P5-1 above. The confirmed isolate isgrown and plated on FOA as before until the loop out of the URA3 markeris confirmed by PCR in one isolate. That isolate is designated strainP-5.

P-6. Integration fragment P6-1 (having nucleotide sequence SEQ ID NO:12) contains the Rhizopus delemar MDH (RdMDH) gene (having thenucleotide sequence SEQ ID NO: 13), an ADHb upstream integration arm,ENO promoter, RKI terminator, URA3 promoter and the first 583 base pairsof the URA3 marker.

Integration fragment P6-2 (having nucleotide sequence SEQ ID NO: 14)contains the Actinobacillus succinogenes FUM (AsFUM) gene (nucleotidesequence SEQ ID NO: 15), the last 568 base pairs of the URA3 marker,URA3 promoter, PGK promoter, PDC terminator and ADHb downstreamintegration arm.

Strain P-5 is simultaneously transformed with each of integrationfragments P6-1 and P6-2 using the standard lithium acetate processdescribed before. Successful transformants are identified by PCR, andgrown and plated until a strain in which the URA3 marker has looped outis identified as before. This strain is designated as strain P-6.

Second Rhizopus delemar MDH integration fragment P6-3 (having thenucleotide sequence SEQ ID NO: 16) contains the Rhizopus delemar MDHgene (having nucleotide sequence SEQ ID NO: 13), ADHb downstreamintegration arm, ENO promoter, RKI terminator, URA3 promoter and thefirst 583 base pairs of the URA3 marker.

Second A. succinogenes FUM (AsFUM) integration fragment P6-4 (havingnucleotide sequence SEQ ID NO: 17) contains the truncated AsFUM gene(nucleotide sequence SEQ ID NO: 15) the last 568 base pairs of the URA3marker, URA3 promoter, PGK promoter, PDC terminator and ADHb upstreamintegration arm.

Strain P-6 is simultaneously transformed with integration fragments P6-3and P6-4, using the standard lithium acetate process described before.Successful transformants are identified by PCR, and grown and plated onFOA as before until a strain in which the URA3 marker has looped out isidentified. This strain is designated as strain P-7.

TABLE 1 Preparatory Strains P-1 through P-7 Strain name DescriptionParent strain P-1 Organic acid tolerant I. orientalis isolate Wild-typeP-2 URA3 deletion (2) P-1 P-3 URA3 deletion (2) P-2 PDC deletion (2) P-4URA deletion (2) P-3 PDC deletion (2) I. orientalis PYC1 insertion atADHa (2) S. pombe MAE insertion at ADHa (2) P-5 URA deletion (2) P-4 PDCdeletion (2) I. orientalis PYC1 insertion at ADHa (2) S. pombe MAEinsertion at ADHa (2) L. mexicana FRD insertion at CYB2b (2) P-6 URAdeletion (2) P-5 PDC deletion (2) I. orientalis PYC1 insertion at ADHa(2) S. pombe MAE insertion at ADHa (2) L. mexicana FRD insertion atCYB2b (2) R. delemar MDH insertion at ADHb (1) A. succinogenes FUMinsertion at ADHb (1) P-7 URA deletion (2) P-6 PDC deletion (2) I.orientalis PYC1 insertion at ADHa (2) S. pombe MAE insertion at ADHa (2)L. mexicana FRD insertion at CYB2b (2) R. delemar MDH insertion at ADHb(2) A. succinogenes FUM insertion at ADHb (2)

Examples 1-9: Integration of Soluble Transhydrogenase

General procedure for producing Examples 1-9: The host strain (asindicated in Table 2 below) is simultaneously transformed with each oftwo integration fragments, as indicated in Table 2 below, using thestandard lithium acetate process described before. The integrationfragments are designed for targeted insertion at the native MAE1 gene ofthe host strain. Integration occurs via three cross-over events: theMAE1 upstream homology, the MAE1 downstream homology and homologybetween portions of the URA3 gene that are present in each of theintegration fragments. Transformants are streaked to isolates and thecorrect integration of the cassette at the MAE1 locus is confirmed byPCR using primers having nucleotide sequences SEQ ID NOs: 80 and 81 toconfirm the 5′-crossover and SEQ ID NOs: 85 and 126 to confirm the3′-crossover. That strain is grown and plated on FOA as before until theloopout of the URA3 marker from an isolate is confirmed by PCR.

The integration fragments used to produce strain Examples 1-9 are asfollows:

Integration Fragment 1A: Left Hand Integration Fragment—Marker Only

Integration fragment 1A, having the nucleotide sequence SEQ ID NO: 18,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis MAE1 open reading frame, an I. orientalis PDC1 promoter,the I. orientalis TAL terminator, the I. orientalis ENO promoter, I.orientalis RKI terminator, URA3 promoter, and the first 582 bp of the I.orientalis URA3 open reading frame.

Integration Fragment 1B: Right Hand Integration Fragment—Marker Only

Integration fragment 1B having the nucleotide sequence SEQ ID NO: 19,contains the following elements, 5′ to 3′: a DNA fragment correspondingto the last 567 bp of the I. orientalis URA3 open reading frame, the I.orientalis URA3 terminator, the I. orientalis URA3 promoter, the I.orientalis TDH3 promoter, the I. orientalis TKL terminator, the I.orientalis PGK promoter, the I. orientalis PDC terminator and a DNAfragment with homology for integration corresponding to the regionimmediately downstream of the I. orientalis MAE1 open reading frame.

Integration Fragment 1C: Left Hand Integration Fragment with the E. coliSthA Gene

Integration fragment 1C, having the nucleotide sequence SEQ ID NO: 20,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis MAE1 open reading frame, an I. orientalis PDC1 promoter,the I. orientalis TAL terminator, the I. orientalis ENO promoter, the E.coli SthA gene (having nucleotide sequence SEQ ID NO: 21), I. orientalisRKI terminator, URA3 promoter, and the first 582 bp of the I. orientalisURA3 open reading frame.

Integration Fragment 1D: Right Hand Integration Fragment with the E.coli SthA Gene

Integration fragment 1D, having nucleotide sequence SEQ ID NO: 22,contains the following elements, 5′ to 3′: a DNA fragment correspondingto the last 567 bp of the I. orientalis URA3 open reading frame, the I.orientalis URA3 terminator, the I. orientalis URA3 promoter, the I.orientalis TDH3 promoter, the I. orientalis TKL terminator, the I.orientalis PGK promoter, the E. coli SthA gene (having nucleotidesequence SEQ ID NO: 21), the I. orientalis PDC terminator and a DNAfragment with homology for integration corresponding to the regionimmediately downstream of the I. orientalis MAE1 open reading frame.

Integration Fragment 1E: Left Hand Integration Fragment with a CodonOptimized E. coli SthA Gene

Integration fragment 1E, having the nucleotide sequence SEQ ID NO: 23,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis MAE1 open reading frame, an I. orientalis PDC1 promoter,the I. orientalis TAL terminator, the I. orientalis ENO promoter, thecodon optimized E. coli SthA gene (having nucleotide sequence SEQ ID NO:24), I. orientalis RKI terminator, URA3 promoter, and the first 582 bpof the I. orientalis URA3 open reading frame.

Integration Fragment 1F: Right Hand Integration Fragment with the CodonOptimized E. coli SthA Gene

Integration fragment 1F, having the nucleotide sequence SEQ ID NO: 25,contains the following elements, 5′ to 3′: a DNA fragment correspondingto the last 567 bp of the I. orientalis URA3 open reading frame, the I.orientalis URA3 terminator, the I. orientalis URA3 promoter, the I.orientalis TDH3 promoter, the I. orientalis TKL terminator, the I.orientalis PGK promoter, the codon-optimized E. coli SthA gene (havingnucleotide sequence SEQ ID NO: 24), the I. orientalis PDC terminator anda DNA fragment with homology for integration corresponding to the regionimmediately downstream of the I. orientalis MAE1 open reading frame.

Integration Fragment 1G: Left Hand Integration Fragment with the A.vinelandii SthA Gene

Integration fragment 1G, having the nucleotide sequence SEQ ID NO: 26,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis MAE1 open reading frame, an I. orientalis PDC1 promoter,the I. orientalis TAL terminator, the I. orientalis ENO promoter, the A.vinelandii SthA gene (having nucleotide sequence, SEQ ID NO: 27), I.orientalis RKI terminator, URA3 promoter, and the first 582 bp of the I.orientalis URA3 open reading frame.

Integration Fragment 1H: Right Hand Integration Fragment with the A.vinelandii SthA Gene

Integration fragment 1H, having the nucleotide sequence SEQ ID NO: 28,contains the following elements, 5′ to 3′: a DNA fragment correspondingto the last 567 bp of the I. orientalis URA3 open reading frame, the I.orientalis URA3 terminator, the I. orientalis URA3 promoter, the I.orientalis TDH3 promoter, the I. orientalis TKL terminator, the I.orientalis PGK promoter, the A. vinelandii SthA gene (having nucleotidesequence SEQ ID NO: 27), the I. orientalis PDC terminator and a DNAfragment with homology for integration corresponding to the regionimmediately downstream of the I. orientalis MAE1 open reading frame.

Integration Fragment 1I: Left Hand Integration Fragment with the S.Cerevisiae Stb5p Gene

Integration fragment 1I, having nucleotide sequence SEQ ID NO: 29,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis MAE1 open reading frame, an I. orientalis PDC1 promoter,the S. cerevisiae Stb5p gene (having nucleotide sequence SEQ ID NO: 30),the I. orientalis TAL terminator, the I. orientalis ENO promoter, the I.orientalis RKI terminator, URA3 promoter, and the first 582 bp of the I.orientalis URA3 open reading frame.

TABLE 2 I. orientalis Insertion Strains Description (in addition tothose in strain P- Integration Parent Designation 7) Fragments strainEx. 1 E. coli SthA insertion at MAE1 (1) 1C/1B P-7 Ex. 2 E. coli SthAinsertion at MAE1 (2) 1D/1A Ex. 1 Ex. 3 A. vinelandii SthA insertion atMAE1 (1) 1G/1B P-7 Ex. 4 A. vinelandii SthA insertion at MAE1 (2) 1H/1AEx. 3 Ex. 5 Codon optimized E. coli SthA insertion at MAE1 1E/1B P-7 (1)Ex. 6 Codon optimized E. coli SthA insertion at MAE1 1F/1A Ex. 5 (2) P-8S. cerevisiae Stb5p insertion at MAE1 (1) 1I/1B P-7 Ex. 7 S. cerevisiaeStb5p insertion at MAE1 (1) 1I/1D P-8 E. coli SthA insertion at MAE1 (1)Ex. 8 S. cerevisiae Stb5p insertion at MAE1 (1) 1I/1F P-8 Codonoptimized E. coli SthA insertion at MAE1 (1) Ex. 9 S. cerevisiae Stb5pinsertion at MAE1 (1) 1I/1H P-8 A. vinelandii SthA insertion at MAE1 (1)

Examples 9-63

Strains P-9 through P-13 are prepared in the same manner as strain P-7,except the L. mexicana FRD gene in each case has been mutated to renderit NADPH-dependent. In each case, the L. mexicana FRD gene having thenucleotide sequence SEQ ID NO: 10 is used as a template to modify thecoding sequence to introduce substitutions of amino acid residues of theputative NADH binding domain of the enzyme.

The FRD gene used to prepare strain P-9 is prepared by performingsite-directed substitutions at amino acids 219 (glutamic acid) and 220(tryptophan) to produce a mutated gene having the nucleotide sequenceSEQ ID NO. 32.

The FRD gene used to prepare strain P-10 is prepared by performing asite-directed substitution at amino acid 417 (glutamic acid) to producea mutated gene having nucleotide sequence SEQ ID NO: 33.

The FRD gene used to prepare strain P-11 is prepared by performing asite-directed substitution at amino acid 641 (aspartic acid) to producea mutated gene having nucleotide sequence SEQ ID NO: 34.

The FRD gene used to prepare strain P-12 is prepared by performingsite-directed substitutions at amino acids 861 (glutamic acid) and 862(cysteine) to produce a mutated gene having nucleotide sequence SEQ IDNO: 35.

The FRD gene used to prepare strain P-13 is prepared by performingsite-directed substitutions at amino acids 1035 (aspartic acid) and 1036(serine) to produce a mutated gene having nucleotide sequence SEQ ID NO:36.

The FRD gene used to prepare strain P-14 is prepared by performingsite-directed substitutions at amino acid 411 of a T. brucei FRD genehaving SEQ ID NO: 42 to produce a mutated gene having nucleotidesequence SEQ ID NO: 37.

Examples 10-18 are made in the same manner as Examples 1-9,respectively, except Examples 10-18 are made starting from strain P-9instead of strain P-7. Examples 10-18 correspond to Examples 1-9,respectively, except the FRD gene in Examples 10-18 is the mutated L.mexicana FRD gene having the nucleotide sequence SEQ ID NO: 32.

Examples 19-27 are made in the same manner as Examples 1-9,respectively, except Examples 19-27 are made starting from strain P-10instead of strain P-7. Examples 23-33 correspond to Examples 1-9,respectively, except the FRD gene in Examples 19-27 is the mutated L.mexicana FRD gene having the nucleotide sequence SEQ ID NO: 33.

Examples 28-36 are made in the same manner as Examples 1-9,respectively, except Examples 28-36 are made starting from strain P-11instead of strain P-7. Examples 28-36 correspond to Examples 1-9,respectively, except the FRD gene in Examples 28-36 is the mutated L.mexicana FRD gene having the nucleotide sequence SEQ ID NO: 34.

Examples 37-45 are made in the same manner as Examples 1-9,respectively, except Examples 37-45 are made starting from strain P-12instead of strain P-7. Examples 37-45 correspond to Examples 1-9,respectively, except the FRD gene in Examples 37-45 is the mutated L.mexicana FRD gene having the nucleotide sequence SEQ ID NO: 35.

Examples 46-54 are made in the same manner as Examples 1-9,respectively, except Examples 46-54 are made starting from strain P-13instead of strain P-7. Examples 46-54 correspond to Examples 1-9,respectively, except the FRD gene in Examples 46-54 is the mutated L.mexicana FRD gene having the nucleotide sequence SEQ ID NO: 36.

Examples 55-63 are made in the same manner as Examples 1-9,respectively, except Examples 55-63 are made starting from strain P-14instead of stain P-7. Examples 55-63 correspond to Examples 1-9,respectively, except the FRD gene in Examples 55-63 is the mutated T.brucei FRD gene having the nucleotide sequence SEQ ID NO: 37.

Examples 64-126—Deletion of Native GPD Gene

Examples 1-63 each are transformed with an integration fragment havingnucleotide sequence SEQ ID NO: 38 using lithium acetate methods asdescribed before. This integration fragment contains the followingelements, 5′ to 3′: a DNA fragment with homology for integrationcorresponding to the region immediately upstream of the I. orientalisGPD1 open reading frame, a PDC transcriptional terminator, the URA3promoter, the I. orientalis URA3 gene, a URA3 terminator, an additionalURA3 promoter direct repeat for marker recycling and a DNA fragment withhomology for integration corresponding to the region directly downstreamof the I. orientalis GPD1 open reading frame. Successful transformantsare selected on selection plates lacking uracil, confirmed by PCR usingprimers having nucleotide sequences SEQ ID NOs: 129 and 130 to confirmthe 5′-crossover and SEQ ID NOs: 131 and 132 to confirm the3′-crossover), and are then grown and plated on FOA as before until astrain in which the URA3 marker has looped out is identified. Thisstrain is then transformed with an integration fragment havingnucleotide sequence SEQ ID NO: 39. This integration fragment containsthe following elements, 5′ to 3′: a DNA fragment with homology forintegration corresponding to the region immediately upstream of the I.orientalis GPD1 open reading frame, the URA3 promoter, the I. orientalisURA3 gene, a URA3 terminator an additional URA3 promoter direct repeatfor marker recycling a PDC transcriptional terminator, and a DNAfragment with homology for integration corresponding to the regiondirectly downstream of the I. orientalis GPD1 open reading frame.Successful transformants are again selected on selection plates lackinguracil and confirmed by PCR, using primers having nucleotide sequencesSEQ ID NOs: 130 and 132) to confirm the 5′-crossover and SEQ ID NOs: 129and 131 to confirm the 3′-crossover). Retention of the first GPD1deletion construct is also reconfirmed by repeating the PCR reactionsused to verify proper integration of SEQ ID NO: 38 above. Confirmedisolates are grown and plated on FOA as before until a strain in whichthe URA3 marker has looped out is identified. The strains resulting fromthe transformations of Examples 1-63 are designated Examples 64-126,respectively.

Example 127-252—Deletion of Native PGI Gene

Integration fragment 5-1 (having SEQ ID NO: 40) for the deletion of thefirst copy of the I. orientalis PGI gene, contains the followingelements, 5′ to 3′: a DNA fragment with homology for integrationcorresponding to the region immediately upstream of the I. orientalisPGI open reading frame, a PDC1 transcriptional terminator, the I.orientalis URA3 promoter, gene, and terminator in succession, followedby an additional URA3 promoter which serves as a direct repeat formarker recycling, and a region immediately downstream of the I.orientalis PGI open reading frame.

Integration fragment 5-2 (having SEQ ID NO: 41) for the deletion of thesecond copy of the I. orientalis PGI gene, contains the followingelements, 5′ to 3′: a DNA fragment with homology for integrationcorresponding to the region immediately downstream of the I. orientalisPGI open reading frame, a PDC1 transcriptional terminator, the I.orientalis URA3 promoter, gene, and terminator in succession, followedby an additional URA3 promoter which serves as a direct repeat formarker recycling, and a region immediately upstream of the I. orientalisPGI open reading frame.

Examples 1-127 each are transformed with integration fragment 5-1 usingthe lithium acetate process described before. Successful transformantsare selected on PGI deletion selection plates lacking uracil (SC −ura,+20 g/L fructose, +0.5 g/L glucose) incubated 3-5 days, confirmed by PCRusing primers having nucleotide sequences SEQ ID NOs: 84 and 85 toconfirm the 5′-crossover and SEQ ID NOs: 72 and 86 to confirm the3′-crossover. Successful transformants are grown and plated on FOA asbefore until a strain in which the URA3 marker has looped out isidentified. In each case, the resulting strain is then transformed withintegration fragment 5-2 in the same manner and successful transformantsselected on PGI deletion selection plates lacking uracil (SC −ura, +20g/L fructose, +0.5 g/L glucose) incubated 3-5 days and confirmed by PCRusing primers having nucleotide sequences SEQ ID NOs: 72 and 84 toconfirm the 5′-crossover and SEQ ID NOs: 85 and 86 to confirm the3′-crossover. A successful deletant in which the URA3 marker has loopedout is again identified as before. The strains resulting from thetransformations of Examples 1-126 are designated Examples 127-252,respectively.

Shake Flask Evaluation for Succinate Production

Example 1-1 is streaked out for single colonies on URA selection platesand incubated at 30° C. until single colonies are visible (1-2 days).Cells from plates are scraped into sterile growth medium and the opticaldensity (OD₆₀₀) is measured. Optical density is measured at wavelengthof 600 nm with a 1 cm pathlength using a model Genesys20spectrophotometer (Thermo Scientific). Dry cell mass is calculated fromthe measured OD₆₀₀ value using an experimentally derived conversionfactor of 1.7 OD₆₀₀ units per 1 g dry cell mass.

A shake flask is inoculated with the cell slurry to reach an initialOD₆₀₀ of 0.1-0.3. Prior to inoculation, the 250 mL baffled shake flaskscontaining 1.75 g/L dry CaCO₃ are sterilized. Immediately prior toinoculating, 50 mL of shake flask medium is added to the dry calciumcarbonate. The shake flask medium is a sterilized, 5.5 pH aqueoussolution of urea (2.3 g/L), magnesium sulfate heptahydrate (0.5 g/L),potassium phosphate monobasic (3.0 g/L), trace element solution (1 mL/L)and vitamin solution (1 mL/L), glucose (120.0 g/L), glycerol (0.1 g/L),2-(N-Morpholino) ethanesulfonic acid (MES) (4.0 g/L). For strainslacking the URA3 gene (URA−) 20 mg/L uracil is added to the media. Thetrace element solution is an aqueous solution of EDTA (15.0 g/L), zincsulfate heptahydrate (4.5 g/L), manganese chloride dehydrate (1.0 g/L),cobalt(II) chloride hexahydrate (0.3 g/L), copper(II)sulfatepentahydrate (0.3 g/L), disodium molybdenum dehydrate (0.4 g/L), calciumchloride dehydrate (4.5 g/L), iron sulphate heptahydrate (3 g/L), boricacid (1.0 g/L), and potassium iodide (0.1 g/L). The vitamin solution isan aqueous solution of biotin (D−; 0.05 g/L), calcium pantothenate (D+;1 g/L), nicotinic acid (5 g/L), myo-inositol (25 g/L), pyridoxinehydrochloride (1 g/L), p-aminobenzoic acid (0.2 g/L).

The inoculated flask is incubated at 30° C. with shaking at 150 rpm for72 hours and taken to analysis. Succinate concentration in the broth atthe end of 72 hours fermentation is determined by gas chromatographywith flame ionization detector and glucose by high performance liquidchromatography with refractive index detector.

Examples 2 through 252 are cultured in shake flasks in similar mannerand found to produce succinate. The succinate concentration in the brothis measured and yield and titer are calculated.

Examples 253-255

Integration fragment P6-2a (having nucleotide sequence SEQ ID NO: 116)contains the I. orientalis FUM (IoFUM) gene (nucleotide sequence SEQ IDNO: 70), the last 568 base pairs of the URA3 marker, URA3 promoter, PGKpromoter, PDC terminator and ADHb downstream integration arm.

Integration fragment P6-4a (having nucleotide sequence SEQ ID NO: 125)contains the I. orientalis FUM (IoFUM) gene (nucleotide sequence SEQ IDNO: 70) the last 568 base pairs of the URA3 marker, URA3 promoter, PGKpromoter, PDC terminator and ADHb upstream integration arm.

Strain P-5 is simultaneously transformed with each of integrationfragments P6-1 and P6-2a using the standard lithium acetate processdescribed before. Successful transformants are identified by PCR, thetransformants are grown and plated on 5FOA plates until a strain inwhich the URA3 marker has looped out is identified as before. Thisstrain is designated strain P-6a.

Strain P-6a is simultaneously transformed with each of integrationfragments P6-3 and P6-4a and using the standard lithium acetate processdescribed before. Successful transformants are identified by PCR, thetransformants are grown and plated on 5FOA plates until a strain inwhich the URA3 marker has looped out is identified as before. Thisstrain is designated strain P-7a.

Strain P-7a is transformed with an integration fragments havingnucleotide sequences SEQ ID NO: 38 and SEQ ID NO: 39, deleting the GPDgene as described with respect to Example 64-126 above. The resultingstrain is named P-8a. Strain P-8a is grown and plated on 5FOA platesuntil a strain in which the URA3 marker has looped out is identified asbefore. The resulting strain is named P-8b.

Construction of Strains 253, 254, and 255

Integration fragment 6-1, having nucleotide sequence SEQ ID NO: 133,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately downstream ofthe I. orientalis GPD1 open reading frame, an I. orientalis ENO1promoter, the E. coli SthA gene (having the nucleotide sequence SEQ IDNO: 24), the I. orientalis PDC terminator, a LoxP site, the I.orientalis PGK promoter, the S. cerevisiae MEL5 gene and terminator(having the nucleotide sequence SEQ ID NO: 134), another LoxP site, anda DNA fragment with homology for integration corresponding to the regiondirectly upstream of the I. orientalis GPD1 open reading frame.

Integration fragment 6-2, having nucleotide sequence SEQ ID NO: 135,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis GPD1 open reading frame, an I. orientalis ENO1 promoter,the E. coli SthA gene (having the nucleotide sequence SEQ ID NO: 24),the I. orientalis PDC terminator, the URA3 promoter, the I. orientalisURA3 gene, an additional URA3 promoter direct repeat for markerrecycling and a DNA fragment with homology for integration correspondingto the region directly downstream of the I. orientalis GPD1 open readingframe.

Integration fragment 6-3, having nucleotide sequence SEQ ID NO: 136contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately downstream ofthe I. orientalis GPD1 open reading frame, an I. orientalis ENO1promoter, the A. vinelandii SthA gene (having the nucleotide sequenceSEQ ID NO: 27), the I. orientalis PDC terminator, a LoxP site, the I.orientalis PGK promoter, the S. cerevisiae MEL5 gene and terminator(having the nucleotide sequence SEQ ID NO: 134), another LoxP site, anda DNA fragment with homology for integration corresponding to the regiondirectly upstream of the I. orientalis GPD1 open reading frame.

Integration fragment 6-4, having nucleotide sequence SEQ ID NO: 137,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis GPD1 open reading frame, an I. orientalis ENO1 promoter,the A. vinelandii SthA gene (having the nucleotide sequence SEQ ID NO:27), the I. orientalis PDC terminator, the URA3 promoter, the I.orientalis URA3 gene, an additional URA3 promoter direct repeat formarker recycling and a DNA fragment with homology for integrationcorresponding to the region directly downstream of the I. orientalisGPD1 open reading frame.

Integration fragment 6-5, having nucleotide sequence SEQ ID NO: 138,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately downstream ofthe I. orientalis GPD1 open reading frame, an I. orientalis ENO1promoter, the P. fluorescens SthA gene (having the nucleotide sequenceSEQ ID NO: 139), the I. orientalis PDC terminator, a LoxP site, the I.orientalis PGK promoter, the S. cerevisiae MEL5 gene and terminator(having the nucleotide sequence SEQ ID NO: 134), another LoxP site, anda DNA fragment with homology for integration corresponding to the regiondirectly upstream of the I. orientalis GPD1 open reading frame.

Integration fragment 6-6, having nucleotide sequence SEQ ID NO: 140,contains the following elements, 5′ to 3′: a DNA fragment with homologyfor integration corresponding to the region immediately upstream of theI. orientalis GPD1 open reading frame, an I. orientalis ENO1 promoter,the P. fluorescens SthA gene (having the nucleotide sequence SEQ ID NO:139), the I. orientalis PDC terminator, the URA3 promoter, the I.orientalis URA3 gene, an additional URA3 promoter direct repeat formarker recycling and a DNA fragment with homology for integrationcorresponding to the region directly downstream of the I. orientalisGPD1 open reading frame.

Examples 253, 254 and 255 are constructed in the following manner.Strain P-8b is co-transformed with the integration fragments listed inthe second column of Table 3. Successful integrants in each case areidentified as blue colonies on selection plates with5-bromo-4-chloro-3-indolyl-alpha-D-galactopyranoside and lacking uracil,and confirmed by PCR. PCR oligos used to test the 3′ and 5′ crossoversof each integration fragment are listed in the third through sixthcolumns of Table 3. In each case, successful transformants are grown forseveral rounds and plated on 5-fluoroorotic acid (FOA) plates toidentify a strain in which the URA3 marker has looped out. Loopout ofthe URA3 marker is confirmed by PCR.

TABLE 3 1^(st) 1^(st) 2^(nd) 2^(nd) integration integration integrationintegration 3′ crossover 5′ crossover 3′ crossover 5′ crossoverIntegration oligos SEQ oligos SEQ oligos SEQ oligos SEQ Strain nameFragments ID ID ID ID Example 6-1 and 6-2 NO: 130 and NO: 131 and NO:130 and NO: 131 and 253 145 143 143 144 Example 6-3 and 6-4 NO: 130 andNO: 131 and NO: 130 and NO: 131 and 254 145 141 141 144 Example 6-5 and6-6 NO: 130 and NO: 131 and NO130 and NO: 131 and 255 145 142 142 144

Table 4 summarizes the genetic modifications to Strains 253, 254 and 255(relative to the wild-type strain):

TABLE 4 Strains 253, 254 and 255 Strain name Description 253 URAdeletion (2) PDC deletion (2) I. orientalis PYC1 insertion at ADHa (2)S. pombe MAE insertion at ADHa (2) L. mexicana FRD insertion at CYB2b(2) R. delemar MDH insertion at ADHb (2) I. orientalis FUM insertion atADMb (2) GPD deletion E. coli SthA insertion at GPD (2) 254 URA deletion(2) PDC deletion (2) I. orientalis PYC1 insertion at ADHa (2) S. pombeMAE insertion at ADHa (2) L. mexicana FRD insertion at CYB2b (2) R.delemar MDH insertion at ADHb (2) I. orientalis FUM insertion at ADHb(2) GPD deletion A. vinelandii SthA insertion at GPD (2) 255 URAdeletion (2) PDC deletion (2) I. orientalis PYC1 insertion at ADHa (2)S. pombe MAE insertion at ADHa (2) L. mexicana FRD insertion at CYB2b(2) R. delemar MDH insertion at ADHb (2) I. orientalis FUM insertion atADHb (2) GPD deletion P. fluorescens SthA insertion at GPD (2)Shake Flask Evaluation for Succinate Production for Strains 253-255

Strains P-8, 253, 254 and 255 are separately evaluated for succinateproduction. In each case, the strain is streaked out for single colonieson plates lacking uracil and incubated at 30° C. until single coloniesare visible (1-2 days). Cells from plates are scraped into sterilegrowth medium and the optical density (OD₆₀₀) is measured. Opticaldensity is measured at wavelength of 600 nm with a 1 cm pathlength usinga model Genesys20 spectrophotometer (Thermo Scientific). Dry cell massis calculated from the measured OD₆₀₀ value using an experimentallyderived conversion factor of 1.7 OD₆₀₀ units per 1 g dry cell mass.

A shake flask is inoculated with the cell slurry to reach an initialOD₆₀₀ of 0.1-0.3. Prior to inoculation, the 250 mL baffled shake flaskscontaining 1.28 g/L dry CaCO₃ are sterilized. Immediately prior toinoculating, 50 mL of shake flask medium is added to the dry calciumcarbonate. The shake flask medium is a sterilized, 4.5 pH aqueoussolution of urea (2.3 g/L), magnesium sulfate heptahydrate (0.5 g/L),potassium phosphate monobasic (3.0 g/L), trace element solution (1 mL/L)and vitamin solution (1 mL/L), glucose (120.0 g/L), glycerol (0.1 g/L),2-(N-Morpholino) ethanesulfonic acid (MES) (4.0 g/L). The trace elementsolution is an aqueous solution of EDTA (15.0 g/L), zinc sulfateheptahydrate (4.5 g/L), manganese chloride dehydrate (1.0 g/L),cobalt(II) chloride hexahydrate (0.3 g/L), copper(II)sulfatepentahydrate (0.3 g/L), disodium molybdenum dehydrate (0.4 g/L), calciumchloride dehydrate (4.5 g/L), iron sulphate heptahydrate (3 g/L), boricacid (1.0 g/L), and potassium iodide (0.1 g/L). The vitamin solution isan aqueous solution of biotin (D−; 0.05 g/L), calcium pantothenate (D+;1 g/L), nicotinic acid (5 g/L), myo-inositol (25 g/L), pyridoxinehydrochloride (1 g/L), p-aminobenzoic acid (0.2 g/L).

The inoculated flask is incubated at 30° C. with shaking at 150 rpm for96 hours and taken to analysis. Succinate and glucose concentrations inthe broth at the end of 96 hours fermentation are determined by highperformance liquid chromatography with refractive index detector.Results are as indicated in Table 5:

TABLE 5 Glucose Average glucose Average succinate after consumptionrate, Succinate after production rate, Strain 96 hr, g/L g/L/hr 96 hr,g/L g/L/hr P-8a 5.46 1.190 57.6 0.60 253 6.20 1.185 88.4 0.92 255 6.101.186 84.1 0.88 257 8.58 1.161 89.0 0.93

As can be seen from the data in Table 5, all strains produce succinate.However, Examples 253-255 produce more succinate, at a 50% greater rate,than Strain P-8a.

Example 256

The URA3 gene is deleted from a wild type strain of S. cerevisiae(CEN-PK 113-7D) to create a strain with a uracil auxotrophy. This strainis called S-1.

Ethanol production is eliminated in S-1 by deletion of the three PDCgenes (PDC1, PDC5, and PDC6), using conventional methods, to produce astrain (S-2) that does not produce ethanol. A pathway from pyruvate tosuccinate is introduced into strain S-2 by the integration of thefollowing exogenous genes driven by strong promoters: the I. orientalisPYC gene, the R. delemar MDH gene, the I. orientalis FUM (fumarase), theL. Mexicana FRD gene, and the S. pombe MAE gene. The various promotersinclude the S. cerevisiae CYC1 promoter, the S. cerevisiae ADH1 promoterand the S. cerevisiae GPD1 promoter.

Strain S-3 is transformed with the E. coli soluble transhydrogenase(SthA) gene (SEQ ID NO: 21) under the control of the S. cerevisiae CYC1promoter. The resulting strain (which still is prototrophic for uracil)is called S-4. Strain S-4 cannot produce ethanol, has an activemetabolic pathway to succinate, overexpresses the solubletranshydrogenase enzyme and is prototrophic for uracil.

After deletion of the PDC genes from S. cerevisiae, it becomes necessaryto supplement the growth medium with a C2 carbon source to supportgrowth. Additionally, glucose is known to suppress growth of S.cerevisiae strains lacking adequate PDC activity. Therefore, Strains S-3and S-4 are grown on a medium containing ethanol as a sole carbon sourceto a suitable cell density in a shake flask. The cells are collected bycentrifugation and the ethanol media discarded. The cells areresuspended in a glucose containing medium in a shake flask andcultivated under aeration at 30° in a stirred shake flask, and succinateformation is monitored until glucose depletion. Strain S-4, whichexhibits transhydrogenase activity, shows improved succinate productioncompared with strain S-3, which lacks transhydrogenase activity.

The invention claimed is:
 1. A fermentation broth comprising at leastone carbon source and recombinant yeast cells engineered to producesuccinate through an active reductive tricarboxylic acid (TCA) pathwayfrom pyruvate or phosphoenolpyruvate, wherein the recombinant yeastcells are genetically modified to express a soluble nicotinamide adeninedinucleotide phosphate (NAD(P)+) transhydrogenase enzyme in the cytosolof the yeast cells by having integrated into their genomes an exogenousgene encoding the soluble NAD(P)+ transhydrogenase enzyme, wherein therecombinant yeast cells are further modified by having integrated intotheir genomes at least one of: (i) an exogenous pyruvate carboxylasegene that encodes an enzyme which catalyzes the conversion of pyruvateto oxaloacetate; (ii) an exogenous malate dehydrogenase gene whichencodes an enzyme that catalyzes the conversion of oxaloacetate tomalate; (iii) an exogenous fumarase gene that encodes an enzyme whichcatalyzes the conversion of malate to fumarate; and (iv) an exogenousfumarate reductase gene which encodes an enzyme which catalyzes theconversion of fumarate to succinate, and wherein the recombinant yeastcells produce more succinate through the active reductive TCA pathway ascompared to a corresponding parent yeast cell lacking the solubleNAD(P)+ transhydrogenase enzyme.